INHERITANCE 741 



Dioecious races. — In these the sexes are in separate clones, all mem- 

 bers of any one clone being of the same sex. The diploid zygote is 

 formed by the union of two haploids of opposite sex and from different 

 clones. By the two maturation divisions, the zygote divides into four 

 cells. Two of these are always of one sex, two of the other. Thus the 

 sexes are segregated at the reduction division, as if sex were determined 

 by a single chromosome pair, one member of which produces one sex, 

 the other the other sex. Sex is determined by the genetic constitution of 

 the clone; sex determination is genotypic. Such dioecious stocks occur in 

 certain races of Chlamydomonas eugametos, of Polytoma pascheri, and 

 of Protosiphon botryoides. 



Suhdioecious races. — In certain races of Chlamydonionas eugametos 

 there is found a modification of the dioecious condition. In any single 

 clone, most of the individuals belong to one sex, a few to the opposite 

 sex. Some clones are prevailingly plus, others prevailingly minus. A num- 

 ber of different matings are possible: 



1. Plus gamete from a prevailingly plus race, minus gamete from a 

 prevailingly minus race. Two of the cells from the zygote yield prevail- 

 ingly plus clones, two yield prevailingly minus clones. The segregation 

 of "prevailingly plus" from "prevailingly minus" therefore occurs at 

 the reduction division; the difference is genotypic. 



2. Plus and minus gametes from a prevailingly plus race. All clones 

 from the zygote are prevailingly plus. 



3. Plus and minus gametes from a prevailingly minus race. All clones 

 from the zygote are prevailingly minus. 



In other crosses, clones from a dioecious race, in which the clones are 

 exclusively of one sex, are crossed with prevailingly plus or prevailingly 

 minus clones from suhdioecious races. The former will be spoken of as 

 "pure" for sex, the latter as "mixed" for sex. 



4. Plus gamete from a race pure for sex, minus gamete from a pre- 

 vailingly minus clone of mixed race. Offspring: two pure plus, two 

 prevailingly minus mixed. 



In other crosses of this type, the results were similar; the "pure" 

 condition segregates from the "mixed" condition at the reduction divi- 

 sion. In general, the two types that unite to produce the zygote separate 

 again at the reduction division; the differences are genotypic. 



The fact that in single clones of the suhdioecious races some indi- 



